EASTERN NECK ISLAND -- Standing in the shallows, a grea blue heron glared at the huge, yellow dump truck that had interrupted its lunchtime fishing.

The truck deposited its 20-ton load of granite, and a power shovel roared to life. Its mantis-like arm lifted boulders one by one and placed them gently atop a rock wall shielding the shoreline and the heron from the wind-whipped waves of the Chesapeake Bay.

Nature reigns at Eastern Neck National Wildlife Refuge, which occupies this 2,200-acre island jutting into the mouth of the Chester River on the Eastern Shore.

With Baltimore's smokestacks visible across the bay, the island is a winter home for thousands of Canada geese, ducks and tundra swans. It also harbors Delmarva fox squirrels and bald eagles, two federally protected endangered species.

The U.S. Fish and Wildlife Service, which owns the island, is spending $2.9 million in a battle to save the island.

Attempting to halt shoreline erosion, government contractors have barged more than 72,000 tons of rock from a quarry near Havre de Grace since April. The rocks are being spread along more than a mile of the refuge's waterfront.

Since the government acquired the island 30 years ago, refuge managers have been watching the steep western shoreline retreat 10 feet a year under the pounding of the bay's waves.

"Every time we'd get a storm, big chunks of bluff would go 'whoosh!' and fall right off," said Thomas A. Goettel, the refuge's manager. A couple acres of land were being lost each year, he estimated.

Chunks of history were washing away, too. The island was a favored Indian hunting and fishing ground and was among the first areas settled by Europeans on the Eastern Shore. An 18th-century graveyard has already eroded. One day a skeleton was exposed in the crumbling cliff, Mr. Goettel said.

Dramatic though it may be, the erosion at Eastern Neck is not uncommon. The bay's 7,300 miles of tidal shoreline is retreating about a foot a year, and much of the mid-bay shoreline is receding 2 feet to 8 feet a year, or more.

Overall, an area the size of the District of Columbia -- about 45,000 acres -- has been lost along the shores of the bay in the last century.

Some scientists think erosion has been worsening, thanks to the rising sea level and the gradual sinking of some waterfront land.

Heavy pumping of ground water and the accumulated weight of sediment running off farmland and development may be causing land to subside about 1.5 millimeters a year, or an inch over the course of about 17 years, according to University of Maryland scientists.

While shoreline erosion eats away at valuable waterfront property and threatens some people's homes, it also is one of the bay's biggest pollution sources. The wind and waves take into the bay millions of pounds of mud, toxic contaminants and life-choking nutrients.

In all, about 4.7 million tons of mud and sand are dumped into the bay every year by shoreline erosion, the U.S. Army Corps of Engineers estimates.

Indeed, wave action puts more sediment into the water than does runoff from construction sites.

Eroding shoreline muddies the water, preventing sunlight from reaching underwater grasses that feed and shelter fish and waterfowl. The mud carries nitrogen and phosphorus, two nutrients that stimulate massive growths of algae, which suck life-supporting oxygen out of the bay's deeper waters.

A Virginia study estimated that shoreline erosion accounts for 5 percent of the nitrogen and 24 percent of the phosphorus entering the lower bay.

At Eastern Neck, whenever a big storm blew across the bay, "You'd get a tremendous plume of muddy water coming off the island," Mr. Goettel said.

That may explain why crabbing has declined there. At one time, as many as 100 cars would park at the refuge on summer days to crab along its western shore, Mr. Goettel said.

But the bay grasses that used to carpet the bottom offshore and provide a haven for crabs vanished a couple of decades ago.

Besides saving the shoreline, the rocks being placed at Eastern Neck may help bring back the grasses, Mr. Goettel said, by reducing wave action in the shallows.

Chianbro Inc., a Landover firm, is using the granite boulders to build a string of breakwaters. Placed end to end about 50 feet apart, the 100-foot long walls are designed to weaken waves before they reach the beach. The 28 barriers, some of them up to 65 yards offshore, girdle the beach.

The beach itself will be armored with a layer of softball-size rocks to shield it from the pounding of waves that might surge over the break waters during storms.

New marshland may be created behind the breakwaters with about 58,000 cubic yards of sand, mud and shells that are to be dredged from the mouth of the Chester River.

Though more expensive than bulkheading, the traditional solution, breakwaters preserve the shallow water environment for fish and vegetation. They also avoid the need to put lumber treated with toxic arsenic into the water.

While it would be too costly to try to halt all shoreline erosion, Dr. Stephen P. Leatherman, director of the University of Maryland's coastal research laboratory, said that the bay's vanishing islands, like Eastern Neck, should be saved.

"People say we should let nature take its course," he said. But he said abandoned, eroding islands like Eastern Neck and Bodkin and Barren farther south in the bay are some of the last refuges from development for the region's waterfowl.